Heretofore the type of aircraft gun sighting device which has been primarily relied upon for aircraft combat missions has been the so-called lead computing optical sight (LCOS), which has been in use in essentially the same form since the latter portion of World War II. An example of an LCOS system is described in U.S. Pat. No. 2,467,831 issued to F. V. Johnson in 1949.
This type of system provides the pilot with a reticle image on an optical head-up display panel. Through use of collimating optics in the sight system, the image of the reticle is made to appear at infinity in the pilot's field of view. The position of the reticle on the display panel is controlled by a two-axis gyro in a manner which is dependent upon the angular velocity of the line of sight to the target and projectile time of flight to the target. As originally conceived, operation of the LCOS system required the pilot to maneuver the attacking aircraft so that the reticle was fixed on the target for some minimum time. At the same time, an accurate estimate of the range of the target aircraft had to be entered into the system.
When the target is being "tracked" by the LCOS reticle and an accurate target range input is available, the attacking aircraft is properly oriented so that the muzzle velocity vector of its gun (appropriately compensated for gravity drop) is in the plane of the velocity vector of the target and is offset at the correct lead angle. Firing of the gun at this time maximizes the likelihood of achieving a hit on the target. The LCOS can also be used without tracking of the target by firing approximately one bullet time of flight before the reticle intercepts the target. This technique however requires considerable skill on the part of the pilot and is useful only when the reticle pip is brought into the immediate vicinity of the target.
Experience has shown that the LCOS system has a high degree of reliability only when the angle-off (angle at which the velocity vector of the attacking aircraft intersects the line of the velocity vector of the target aircraft) is relatively small (e.g., less than 30.degree.), when the rate at which the attacking aircraft is closing on the target is low, and when a relatively accurate measurement of the target range is available.
Fighter aircraft combat conditions, however, have changed dramatically since the era when the LCOS system was developed and extensively exploited. Air-to-air missile systems generally have replaced guns as the principal aircraft armament in the post-Korean War period, i.e., the late 1950's and 1960's. It was throught that air-to-air missiles would render the need for gun systems obsolete. However, aircraft combat experience during the 1960's demonstrated that certain combat situations could be encountered in which a gun system could be utilized as a highly effective complement to an air-to-air missile system.
In relatively close-in combat, it has been found that a highly maneuverable aircraft with a gun system can obtain an advantage over a higher speed, less manueverable aircraft equipped only with missiles. A situation which has been encountered is one where the higher speed aircraft initially attacks by executing a missile pass. The more maneuverable target aircraft, detecting the missile launch, turns tightly into the direction of the attack, thereby avoiding the missile. Without a gun system the attacking aircraft has no way of further pursuing the tactical advantage he enjoys at that moment.
However, if the attacking aircraft is also equipped with a gun system a significant advantage is gained. Again consider the above-described attack situation. When the target aircraft turns to avoid the missile, the attacking aircraft, still being in a trailing position with respect to the target aircraft, has, for a relatively short period of time, an excellent opportunity to make a gun attack on the target.
However, because of the maneuvering positions of the two aircraft under these conditions, a gun attack can usually be made only at a high angle-off between the attacking and target aircraft. Typically, an angle-off of 90.degree. or more can be experienced. This means that the attack will be characterized by a high rate of change of the line of sight to the target aircraft by a high range closing rate and by a very narrow target opportunity "window".
In such an attack situation an LCOS system is of limited value due to the large angle-off and the extreme dynamics involved. Furthermore, the ability to determine target range through radar tracking under such conditions is highly limited for a number of reasons. A reliable radar lock may not be achievable or maintainable due to the high crossing velocity of the target and, furthermore, it may not be desirable from a security standpoint for the attacking aircraft to emit radar radiation when in such a combat situation.